1. Field of the Invention
This invention relates generally to a high presure metal vapor lamp, and more particularly to such a lamp having an arc tube started by means of a glow starter and a radioactive substance provided in the arc tube.
2. Description of the Prior Art
High pressure metal vapor discharge lamps such as metal halide discharge lamps, have been used for their high luminous output and high luminous efficiency. Presently such lamps need their own specially designed ballasts because they have a high starting voltage compared to mercury lamps. Recently, a metal halide lamp which is started with a conventional and inexpensive mercury ballast has been developed. One method to start a lamp with a mercury lamp ballast reduces the starting voltage by using a neon (Ne) gas as a starting gas in the arc tube. That is, by using a well known Penning gas formed mainly of neon gas i.e. Ne (99%)--Ar (1%) or Ne (99%)--Kr (1%), it is possible to start such a lamp with a mercury lamp ballast because the starting voltage is reduced to about 150 V.
Although it is possible to reduce the starting voltage by using neon gas, some disadvantages are evident. Namely, the thermal conductivity of neon is larger than that of other gases such argon, krypton or xenon, so the luminous efficiency using mainly neon gas is reduced by five percent or more compared to using argon gas. Moreover, the use of neon gas results in early blackening of the arc tube and shortens the lamp life.
Accordingly, it is desirable not to use neon gas as a starting gas. So, another starting means using a glow starter has been considered. A lamp using a glow starter is known from Japanese Patent Disclosure No. 52-101,876 or U.S. Pat. No. 4,117,370. Such a lamp, by using a glow starter, easily produces high pulse voltages necessary for starting, while also being simple and inexpensive to manufacture.
However, a glow starter starting operation may be repeated again and again till the lamp actually begins to start. Such repetition of the starting operation shortens the life of a glow starter. But if there are no initial electrons provided as a seed to initiate discharge in the arc tube, it is extremely difficult to start the arc tube in spite of generating a high pulse voltage during glow starter starting operation. Namely the absence of initial seed electrons shortens the life of a glow starter.
On the other hand, it is known to use an emitter, such thorium oxide or thorium metal, as a starting seed. But thorium has a long half-life, i.e. 1.4.times.10.sup.10 years, and it is therefore necesary to provide a large amount of thorium to assure electron release. Moreover, thorium reacts with other sealing metals, such as iodine, and this rapidly reduces the luminous efficiency.